Uranium hexamethoxide and mixed methoxyuranium(VI) fluorides: Facile syntheses from UF₆ and a nuclear magnetic resonance investigation of structure and chemical dynamics

Efficient, one-step scalable syntheses of uranium hexamethoxide, U(OCH₃)₆, starting from readily available UF₆ and NaOCH₃ or CH₃Si(OCH₃)₃ are described. In addition, the reaction of appropriate quantities of (CH₃)₃SiOCH₃ or U(OCH₃)₆ with UF₆ produces a series of mixed methoxyuranium(VI) fluorides, U(OCH₃)_nF_6-n, n = 1-5, the degree of substitution being determined by control of stoichiometry. Characterization of the complexes by both ¹H and ¹⁹F NMR indicates that all the species possess a monomeric, six-coordinate geometry and undergo rapid, intermolecular ligand exchange. A surprisingly large solvent and temperature dependence of the ¹⁹F chemical shifts is interpreted in terms of charge-transfer complex formation. The solution-phase electronic spectrum of U(OCH₃)₆ is interpreted in terms of both ligand- and solvent-to-metal charge transfer.